shadertoy preview runs the native C++ renderer behind a small local web server.
The browser is a viewer/controller; it does not execute a second WebGL renderer.
Native PNG frames are pushed as binary WebSocket messages and drawn to a canvas;
there is no per-frame HTTP image polling. ShaderToy.toml, shader sources, and
assets are watched for changes.
Shader/source edits use an include-aware dependency graph. A successful source
reload recompiles only affected passes and leaves existing render targets and
feedback history intact. Manifest/asset changes take the conservative full
project reload path. Any failed reload keeps the last successful render alive
and exposes the compile/load error.
The preview supports final Image and intermediate 2D buffer views, pause/reset,
frame stepping, time scale, resolution changes, and mouse forwarding.
On Linux the native preview uses the same surfaceless EGL path as check/render, so it can run without DISPLAY or WAYLAND_DISPLAY.
Input recording and replay
--------------------------
Record shader-affecting preview controls together with exact rendered
iFrame/iTime/iTimeDelta/iFrameRate markers:
shadertoy preview --record target/repro.strec
Reproduce the recorded timeline headlessly:
shadertoy replay target/repro.strec -o target/replay.png
Recordings contain a SHA-256 fingerprint of the manifest, expanded shader
sources, and assets. Replay refuses a changed project by default; use
--allow-project-changes only when comparing behavior intentionally. Any watched
project-file edit while recording invalidates that recording, because a single
replay cannot reproduce multiple source revisions; restart preview recording
after the edit settles.